Synopsis:
Developing La Niña
conditions are expected to continue during the next 3-6 months.

Equatorial
SST anomalies greater than +0.5ºC were restricted to the region between
Indonesia and 165ºE during December, while negative anomalies less than
–0.5ºC were observed at most locations between the date line and the South
American coast (Fig. 1). By the end of the month the
SST departures were negative in all of the Niño regions (Fig.
2).
During the last several months surface and subsurface temperature anomalies
have decreased in the region between 180ºW and the South American coast (Fig.
3). During the same period persistent stronger-than-average low-level
equatorial easterly winds were observed over the central Pacific.Since early November there has been a persistent pattern of enhanced
tropical convection near
130ºE (Indonesia) and suppressed convection near the date line (180ºW). Collectively, the present oceanic and atmospheric anomalies are
consistent with the development of La Niña
conditions in the tropical Pacific.

Over the
past several months most of the statistical and coupled model forecasts have
trended towards cooler conditions in the tropical Pacific through mid-2006.
The spread of the most
recent statistical and coupled model
forecasts (weak La Niña to ENSO-neutral) indicates
some uncertainty in the outlooks (Fig. 4). However, current conditions
(stronger-than-average easterly winds over the central equatorial Pacific)
and recent cooling trends in observed oceanic conditions support the
continuation of La Nina conditions in the tropical Pacific during the next
3-6 months.

Based
on current conditions in the tropical Pacific, the most recent SST
predictions,and on results from
historical studies on the effects of cold episodes, we expect
wetter-than-normal (drier-than-normal) conditions to prevail over
Indonesia
(central equatorial Pacific) during the remainder of the NH winter. That
pattern of tropical precipitation favors a northward shift in the position of
the jet stream over the eastern North Pacific during winter, which is usually
accompanied by drier-than-normal conditions over southern
California
and
Arizona. However, given the late onset
of La Niña there is considerable uncertainty as to whether or not typical La
Niña impacts will be experienced in the West during the remainder of NH
winter.

This discussion is a consolidated effort of NOAA and its funded institutions. Weekly updates for
SST, 850-hPa wind, OLR and features of the equatorial subsurface thermal structure are available on the Climate
Prediction Center web page at http://www.cpc.ncep.noaa.gov (Weekly Update). Forecasts for the evolution of El Niño/La Niña are updated monthly in the
Forecast Forum section of CPC's Climate
Diagnostics Bulletin. The next ENSO Diagnostics Discussion is scheduled for